Abstract: The presence of underlying mined-out layer and developed joints have an important impact on the fragmentation process of landslide collapse. In order to further explore the cutting and fragmentation effects and characteristics of joints on rock masses, based on UAV aerial photography and field investigations of debris flows in the Madaling landslide, the particle discrete element method was used to simulate the flow force crushing process of landslide debris with joints in the underlying layer. Changes in the quantity and particle size distribution of fragmented bodies were analyzed. The conclusions are as follows: 1. The development process of debris flow in the Madaling landslide can be summarized as the following: trailing edge tension fracture, stepped creeping tension fracture, shear deformation, slip surface connection, and overall failure of the sliding mass. The collapse of joints and underlying layer promotes the failure and fragmentation process of the sliding mass; 2. Fragmentation occurs during both the failure and movement deposition processes of the sliding mass, with fragmentation dominating during movement deposition; 3. Results fitted with the Weibull dual-parameter model show continuous increase in fine particle fragmentation within the sliding body, ultimately resulting in predominantly medium to small particle size fragmentation in the deposited mass, demonstrating the phenomenon of fragmentation and disintegration during the movement and deposition of the sliding body. This study provides new insights for the analysis of the fracture mechanism of the landslide debris flow with joints in such underlying layer, and proves that the effect of the caving and joint cutting of the underlying layer on the rock mass fracture has a certain guiding significance for the prevention and control of landslide debris flow disasters in areas with similar geological conditions.